Zinc oxide is used commercially in the formulation of rubber products, and as a feed stock for chemicals, pharmaceuticals, catalysts, fertilizers and many other products. Different characteristics may be required for different applications. Maximum concentrations of certain particular contaminants are important in some applications. In addition to purity, the physical properties of a commercial zinc oxide product, such as its particle size and shape, as well as its surface area, may also be of importance for certain applications.
Zinc occurs usually in sulphide mineral deposits. Conventionally, purified zinc oxide may be commercially produced by re-oxidizing metallic zinc which has been recovered by sulphuric acid leaching of roasted zinc sulphide concentrates followed by electrowinning. Zinc oxide may also be commercially produced by controlled oxidation of the zinc vapour which is produced by smelting roasted zinc sulphide concentrates that have been mixed with coal.
Neither of these techniques is used for the production of high surface area zinc oxide which is desirable for some commercial applications.
U.S. Pat. No. 387,688 which issued to Low on Aug. 14, 1888, teaches that zinc may be recovered from zinc ore by roasting the ore and subsequently subjecting the ore to repeated leachings in cold aqueous sulphur dioxide solution. Zinc sulphite may then be recovered from the leachate by boiling.
U.S. Pat. No. 878,866, which issued to Sulman on Jan. 7, 1908, teaches a process wherein calcined zinc sulphides are leached in aqueous sulphur dioxide solution. Zinc oxide is added to the resulting zinc bisulphite solution in order to precipitate zinc monosulphite.
U.S. Pat. No. 1,110,660, which issued to Vadner on Sep. 15, 1914, describes the leaching of zinc oxide bearing ores and calcined concentrates in sulphurous roaster gases dissolved in water. Copper is precipitated out by passing the resulting solution over iron. Oxidation and carbonate precipitation are used to remove iron and other impurities from the solution. Zinc is precipitated as basic zinc carbonate.
U.S. Pat. No. 1,919,947, which issued to Johnston on May 7, 1929, teaches a process for treating oxides containing zinc, cadmium and copper with aqueous sulphur dioxide to obtain a bisulphite solution. Cadmium and copper are claimed to be precipitated from the zinc bisulphite solution by the addition of zinc oxide. The zinc monosulphite obtained is calcined to produce zinc oxide and sulphur dioxide.
None of the processes disclosed in the above mentioned patents disclose the techniques of the present invention which produce a purified zinc oxide product.